The present invention is in the field of protection of electrical conductors from damaging electrical overloading and, more particularly, providing such protection with solid-state power controllers (SSPC's).
In certain electrical power distribution systems, traditional thermal circuit breakers are being replaced with SSPC circuit protection. SSPC circuit protectors may be provided with periodically sampled data relating to current flow in a conductor and may then perform periodic calculations based on the data. A product of each of the calculations is compared with a tripping threshold to determine if current should be interrupted to prevent damage. The tripping threshold is derived from a simulation of traditional bimetallic thermal circuit breaker characteristics.
Industry wire-protection safety standards are based on characteristics of traditional thermal circuit breakers. For example, an industry standard may specify that if an insulated 14 gauge wire is exposed to a current of 20 amps for a period of 10 seconds then a 15 amp thermal circuit breaker should trip. The majority of existing aircraft loads are also designed to comply with such a standard. A bimetallic element within a thermal circuit breaker acts as an energy storage device. When accumulated energy within the breaker reaches a particular value, it trips. Trips at various electricity input levels, as a function of time, may be plotted as a constant energy trip curve.
As a result SSPC circuit protectors are often designed so that they perform comparisons against a continuum of parameters that simulate, as close as possible, the parameters of industry standards. In that regard, the SSPC circuit protectors may be considered to operate with a “simulation trip curve” that simulates a thermal circuit breaker trip curve. As a first approximation for a simulation trip curve, prior-art SSPC circuit protectors have employed a relationship i2t, where i is current and t is time. This relationship provides a simulation trip curve that corresponds in many but not all respects to a thermal circuit breaker trip curve. A typical mismatch of the prior-art trip curves may be illustrated in graph 100 in prior-art FIG. 1.
In FIG. 1, the prior-art simulation trip curve 102 may represent an energy balance between heat generated within a conductor due to electrical resistance and the energy leaving the conductor to an ambient environment. In very short times (t<2 sec), heat generation effects (i2t) dominate and the prior-art simulation trip curve 104 closely follows the thermal circuit breaker trip curve 102. At very long times, a steady state “Never Trip” threshold is achieved at (t>100 sec). However, in an intermediate time frame (2<t<100 sec), convection heat flows may have a bearing on how much energy accumulates in the conductor. The simulation trip curve 104 based on i2t modeling does not take account of convection heat transfer. As a result the curves 102 and 104 may diverge.
This divergence presents several problems. The first is that a gap is created between the protection provided by a thermal circuit breaker and an SSPC circuit protector that is based on the simple i2t model. Specifically the gap develops in the 2<t<100 sec time frame. A load which lies in the region bounded by the I2t curve 104, a Never Trip line 106 and the thermal circuit breaker trip curve 102 may cause opening of an SSPC circuit protection switch. A protection switch would remain closed if the circuit were protected by a traditional thermal circuit breaker. In other words, load in the bounded region above described may cause nuisance trips.
The second problem is one of modelling a correlation between trip curves 102 and 104 as a function of ambient temperature. Due to the fact that a thermal circuit breaker will absorb more energy at colder ambient temperatures and less at hotter ambient temperatures, the thermal circuit breaker trip curve 102 may shift to the left as ambient temperatures rise and to the right as they drop. Correlation of these effects may provide for optimum sizing of wire and thermal circuit breaker combinations. But such correlation is not readily achievable when the simple I2t model is used for the simulation trip curve 104.
Simple i2t trip curve models are established with no dependence on ambient temperature. Therefore, in order to ensure that industry safety standards are not compromised, wire sizes must be selected based on the hottest ambient temperature to which a wire may be exposed. The effect is that a wire size used in a circuit element with prior-art SPPC circuit protection may be larger than a wire size for an equivalent circuit element protected with a thermal circuit breaker.
Various attempts have been made in the prior-art to refine modeling techniques for simulation trip curves. One such technique is disclosed in US Patent Application 2007/0014066. In this disclosure, a first-order thermal model of a wire is used as a basis for determining when and if current should be interrupted within the wire. The thermal model analyzes conduction-based heat flow, but it does not take into consideration radiation and convection heat flows.
As can be seen, there is a need to provide an SSPC based circuit protection system that may operate with characteristics that are in close correspondence with those of a thermal circuit breaker. Additionally, there is a need to provide such a system that takes into account ambient temperature condition in which a protected circuit element operates.